The present invention relates to a surface treatment method of a metal material which is used as a process before an electrodeposition coating process.
In a coating process of automotive vehicles or the like, a chemical conversion treatment is generally applied to a workpiece to be coated (metal material) before a cationic electrodeposition coating. In this chemical conversion treatment, a zinc phosphate-based treatment agent comprising a primary component of zinc phosphate is often used as a chemical conversion treatment agent. The workpiece subjected to the chemical conversion treatment using the zinc phosphate-based treatment agent can obtain excellent electrodeposition coatability (excellent film thickness characteristic of a coating film) in the cationic electrodeposition coating process. However, the zinc phosphate-based treatment agent has a problem in that phosphate ions thereof may cause eutrophication. Moreover, the chemical conversion treatment using the zinc phosphate-based treatment agent may cause a problem of production of sludge to be wasted. With a view to solving these problems, there has been proposed the chemical conversion treatment agent which comprises: at least one selected from the group consisting of zirconium, titanium and hafnium; fluorine; and a water-soluble resin, as disclosed in U.S. Pat. No. 7,510,612, for example.
However, in case a workpiece is subjected to a chemical conversion treatment using a chemical conversion agent comprising a primary component of zirconium (zirconium compound) or the like, a chemical conversion film (ZrO2 or the like), which has a relatively small number of local low-resistance areas i.e., a relatively low electrical conductivity, compared with a chemical conversion film formed using the zinc phosphate-based treatment agent, is formed on a surface of the workpiece. Accordingly, in an electrodeposition coating process, a relatively high voltage is applied between an anode and a portion of the workpiece adjacent to the anode (an outer panel of a vehicle body), whereas a relatively low voltage is applied between the anode and another portion of the workpiece far from the anode (an inner panel of the vehicle body), as a phenomenon specific to the electrodeposition coating process. Herein, the deposition amount of coating film may decrease in the portion of the workpiece far from the anode which belongs to a low voltage-applied region. Thus, in case the chemical conversion agent comprising the primary component of zirconium (zirconium compound) or the like is used, the deposition amount of coating film may improperly decrease in the portion of the workpiece far from the anode (the inner panel of the vehicle body) which belongs to the low voltage-applied region, compared with a case of the zinc phosphate-based treatment agent being used (see FIG. 3).